The present invention relates to a process for the preparation of starch-containing polymer dispersions, the starch-containing polymer dispersions themselves and their use.
Polymer dispersions which contain starch or starch derivatives as protective colloids for stabilizing polymer particles are used as sizes and coating compositions for paper. Such polymer dispersions are obtained as a protective colloid by copolymerization of ethylenically unsaturated monomers by free radical emulsion polymerization in water as a solvent in the presence of starch or starch derivatives, which may have been oxidatively, hydrolytically or enzymatically degraded. Peroxide initiators are used.
EP-A 0 735 065 describes the preparation of amphoteric polymer dispersions in the presence of starch and/or starch derivatives by two-stage polymerization. In a first process stage, a monomer mixture comprising ethylenically unsaturated monomers is polymerized in the presence of starch degraded enzymatically and hydrolytically and/or by acid hydrolysis, peroxide, such as H2O2, being initially taken as a water-soluble free radical initiator with the starch solution or being metered in in combination with reducing components in a continuous feed process. The dispersion obtained in the first stage is polymerized with a further monomer mixture which, in addition to the ethylenically unsaturated monomers A, contains nitrogen-containing monomers B having a basic character. Preferably used free radical initiators are redox systems comprising peroxides and/or hydroperoxides as oxidizing agents and a reducing component, the reducing components being initially taken together with the dispersion prepared in the first process stage and the oxidizing component being metered in simultaneously with the monomer feed.
EP-A 0 307 816 describes the preparation of a paper size, in which the starch in aqueous solution is degraded first enzymatically and then oxidatively in the presence of hydrogen peroxide. Hydrogen peroxide, as free radical initiator, is added all at once to the aqueous solution of the degraded starch and then an emulsion of acrylonitrile and N-butyl acrylate is added continuously.
Simultaneously with the monomer feed, further hydrogen peroxide is continuously metered in separately.
DE-A 36 27 494 describes the preparation of a paper size based on finely divided aqueous dispersions of copolymers of (meth)acrylonitrile, acrylates and, if required, further ethylenically unsaturated monomers in an emulsion polymerization. The copolymerization of the monomers is effected in aqueous solution of a degraded starch in the presence of an initiator containing peroxide groups. In a first stage, a part of the monomers is initially taken together with initiator solution in the aqueous starch solution and is polymerized. Thereafter, in a second stage, the remainder of the monomers and the initiator solution are added continuously or in portions and are polymerized.
The polymer dispersions obtainable by the processes described above are still in need of improvement with regard to their sizing effect.
It is an object of the present invention to provide paper sizes and coating compositions based on starch-containing polymer dispersions having an improved sizing effect.
We have found that this object is achieved by a process for the preparation of a polymer dispersion by copolymerization of from 20 to 70 parts by weight of a monomer mixture comprising
a) from 2.5 to 100% by weight of one or more (meth)acrylates of monohydric, saturated C3-C8-alcohols as monomers A and
b) from 0 to 97.5% by weight of one or more further ethylenically unsaturated monomers as monomers B
in 100 parts by weight of an aqueous solution which contains from 5 to 35% by weight of a starch and/or of a starch derivative by emulsion polymerization in the presence of an initiator containing peroxide groups, in which the initiator is metered in simultaneously with the monomers, wherein a first amount of initiator is metered in over a first feed period of from 5 to 60 minutes and a second amount of initiator is metered in over a second feed period of from 30 to 120 minutes, the first amount of initiator being greater than the second amount of initiator.
We have found, surprisingly, that a substantial improvement in the sizing effect of the prepared polymer dispersion results if the starch is oxidatively degraded during polymerization and over the total polymerization period by the peroxide initiator. If a substantial oxidative degradation is carried out over the total polymerization period, the polymer dispersion becomes unstable due to destruction of the protective colloid, which leads to undesirable filtration residues. However, as a result of the novel two-stage initiator feed with less initiator in the second stage, high sizing efficiency of the polymer dispersion can be realized without formation of filtration residue.
Suitable initiators containing peroxide groups are hydrogen peroxide and organic peroxide, hydroperoxides and peroxodisulfates, for example cumyl hydroperoxide, tert-butyl hydroperoxide, sodium peroxodisulfate, potassium peroxodisulfate or ammonium peroxodisulfate.
A preferred initiator is hydrogen peroxide. Hydrogen peroxide can be used as an initiator alone or in combination with a heavy metal salt and, if required, a further reducing agent. Preferably, hydrogen peroxide is used in combination with a heavy metal salt, preferably iron(II) sulfate. Suitable further reducing agents are, for example, ascorbic acid, sodium bisulfite or sodium dithionite. Since even the starch present in aqueous solution itself acts as a reducing agent, the use of a further reducing agent is less preferable. The heavy metal salt is initially taken in general together with the aqueous starch solution.
The monomer mixture is metered continuously into the aqueous solution containing the starch and/or the starch derivative and, if required, the heavy metal salt, simultaneously a first amount of initiator being metered in over a first feed period of from 5 to 60, preferably from 10 to 30, minutes and a second amount of initiator being metered in over a second feed period of from 15 to 180, preferably from 30 to 120, minutes. In general, the first amount of initiator is from 2 to 4 times the second amount of initiator. If the initiator is hydrogen peroxide, the first amount of initiator is preferably from 2 to 4% by weight and the second amount of initiator from 0.5 to 1% by weight, based on the weight of the total amount of monomers metered in.
In general, from 20 to 70, preferably from 40 to 50, parts by weight of a monomer mixture comprising the monomers A and, if required, B in 100 parts by weight of the aqueous solution which contains from 5 to 35, preferably from 10 to 25, % by weight of starch and/or starch derivatives are copolymerized.
Monomers A are, for example, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, neopentyl acrylate, n-hexyl acrylate, cyclohexyl acrylate and 2-ethylhexyl acrylate. Preferred monomers A are n-butyl acrylate, isobutyl acrylate and tert-butyl acrylate and mixtures thereof.
Suitable further ethylenically unsaturated monomers B are, for example, ethylenically unsaturated C3- to C5-carboxylic acids, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid and monoesters of maleic acid, styrene, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, further C1- to C18-alkyl (meth)acrylates differing from the monomers A, such as methyl acrylate, ethyl acrylate, decyl acrylate, palmityl acrylate and stearyl acrylate, and basic compounds, such as dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl acrylate, N-vinylimidazoline and N-vinyl-2-methylimidazoline.
In a preferred embodiment,
a) from 10 to 50, particularly preferably from 20 to 40, % by weight of n-butyl acrylate are copolymerized as monomer A with
b) from 50 to 90, particularly preferably from 60 to 80, % by weight of styrene as monomer B,
the sum of the components a) and b) being 100% by weight.
The copolymerization of the monomers A and, if required, B is effected in an aqueous medium in the presence of a starch or of a starch derivative. Suitable starches are natural starches, such as potato, wheat, rice, tapioca and corn starch. Suitable starch derivatives are chemically modified starches, such as hydroxyethylstarch, hydroxypropylstarch or quaternized aminoalkyl-containing starches. The starches or starch derivatives can be subjected to an acidolytic or enzymatic degradation before the beginning of the polymerization reaction. It is also possible to use oxidatively degraded starches which, if required, are further degraded acidolytically or enzymatically before the beginning of the polymerization reaction.
In a preferred embodiment of the invention, oxidatively degraded starch is initially taken in aqueous solution and enzymatically degraded. The enzymatic degradation is stopped and an aqueous heavy metal salt solution is added. A monomer emulsion, containing the monomers A and, if required, B in an emulsifier is allowed to run continuously into this solution. Simultaneously with the monomer addition, the initiator feed is begun. The initiator feed may end with the monomer feed or may be continued beyond it.
However, it is also possible not to start the initiator feed until after the beginning of the monomer feed. What is important is that altogether a smaller amount of in itiator is metered in over the second feed period than over the first feed period.
The polymerization temperature is in general from 60 to 100xc2x0 C., preferably from 75 to 95xc2x0 C. at normal pressure.
The present invention also relates to the polymer dispersions obtainable by the process described above and to their use as paper sizes and coating compositions.
The Examples which follow illustrate the invention.